What is the definition of intensity in ultrasound physics?

• A. The rate at which energy is transferred
• B. Power divided by cross-sectional area of the beam
• C. Amplitude of the wave
• D. The speed of sound in the medium

Answer: (B)

Intensity in ultrasound is the energy transferred per unit area per unit time. It is defined as the beam’s power divided by its cross-sectional area, I = P/A. This means energy flowing through a smaller area with the same power results in higher intensity, while spreading the same power over a larger area lowers the intensity. For example, delivering 2 watts through a 2 cm^2 beam gives 1 W/cm^2 of intensity; halving the beam area to 1 cm^2 with the same power would raise intensity to 2 W/cm^2.

Power is the rate of energy transfer, so it’s not the same as intensity, which adds the per-area aspect. The amplitude of the wave relates to pressure or particle motion, not directly to energy per unit area, and the speed of sound is a property of the medium that influences energy transport but does not define intensity itself.